This study reports for the first time the phenomenon of supramolecular solvent formation based on alkyl polyglucoside as an amphiphile and primary alcohol as a coacervation agent. The physical properties (density, kinematic viscosity, phase diagram for ternary system) of the supramolecular solvent were investigated, and a mechanism for its formation was proposed. A green and simple microextraction procedure for preconcentration and determination of phthalates in baby foods packaged in plastic packaging was developed as proof-of-concept example. The microextraction procedure assumed separation of analytes from solid phase sample in micellar solution of decyl glucoside and in situ formation of supramolecular solvent for analytes preconcentration after addition of n-heptanol. The determination of phthalates in obtained extracts was implemented by high-performance liquid chromatography with UV–Vis detection. The limits of detection, calculated from a blank test based on 3σ, were determined to be 10 μg kg−1 for dimethyl phthalate, diethyl phthalate, di-n-butyl phthalate, and di-n-octyl phthalate. The developed procedure did not require filtration of sample suspension, and assumed the use of green and biodegradable substances for the supramolecular solvent formation across a wide pH range.